Geologists of Jackson Hole Field Trip to Northern Scotland May 2015
Total Page：16
File Type：pdf, Size：1020Kb
Geologists of Jackson Hole Field Field Trip to Northern Scotland May 2015 By John Willott & Mike Adler Outline • Formation of Scotland – Plate Tectonics • Geology of the Northwest Highlands • Geology of Skye Geologic Map of Great Britain Map of Northern Scotland Lewisian gneiss, Scotland, 2012 Gneiss, south coast Newfoundland, 1987 Early Geologic Concepts Three Ages of Rocks Primary Secondary Tertiary Older Rock Is Always Under Younger Rock Thrust Faults Not Recognized Why Study Northern Scotland? 3 Billion years of rocks No ground cover Everything is visible Birthplace of Modern Geology • The Scottish highlands have been extensively studied for over 200 years and have been the birth place for key elements of modern geological understanding • James Hutton in 1795 proposed that earth’s geology evolved slowly over vast time periods and not by a series of catastrophes over thousands of years and is regarded as the founder of modern geology and he viewed “the present is the key to the past” • In 1830 this was popularized and named the “Principle of Uniformitarianism” by Charles Lyell in his book “The Theory of Geology” • Lyell’s work strongly influenced Charles Darwin who viewed evolution as biological uniformitarianism which occurred imperceptibly over many generations • Charles Lapworth in 1881 showed that folding could replicate the same beds many times • Ben Peach and John Horne in 1907 were the first to demonstrate thrust faulting • The Highlands have also been the source of a 60 year bitter debate from 1840 to 1905 in the highest geological circles of Great Briton called the “Highlands Controversy” Main Rock Types in the Northern Highlands • Lewisian Gneiss – Precambrian(no fossils) high grade metamorphic rock made from mudstone and shale (high grade meaning subject to the highest temperatures, pressures for the longest times forming large crystals) – Types depend on the content of specific minerals and is often banded with layers of different mineral content – It named after the outer Hebridean island of Lewis which is completely covered by the gneiss – It is 3 Billion years old making it some of the oldest rock on earth • Torridonian Sandstone – Coarse grain sedimentary sandstone containing a conglomerate mix of other rock types – Precambrian(1 Billion years in age & no fossils) and is not metamorphosed – Is deposited unconformally (2000Myr) on denuded rolling gneiss with small hills and valleys – This unconformity is one of the most striking features of the North West Highlands Main Rock Types in Northern Highlands • Moine Schist – Precambrian medium grade metamorphic rock formed from mudstone and shales – Has medium grained mica flakes in a preferred sheet like orientation called schistocity – Has a preponderance of lamellar (flat, planar) minerals such as micas, chlorite, talc, and graphite also containing quartz – Its name Moine came from A'Mhoine on the north coast which is a region of peat bogs – At the thrust zone the schist can be metamorphosed by the shearing action from the thrust and are then referred to as Moine mylonites – Sedimentary Cambrian Rocks • Durness Limestone named after the town of Durness at the northwest corner of Scotland is fairly pure calcium carbonate with fossils present(less than 500 Myr) • Salterella grits are a band of quartzite and grit at base changing to carbonate dolomite with worm pipes up to an inch in diameter ,forming an escarpment between fucoid beds and limestone above • Fucoid beds vary in composition with some containing carbonates and others more shale like with a rusty color • Quartzite 500’ in thickness ,at bottom layered coarse quartz and feldspar and at top “pipe rock”,fine grained quartz containing vertical worm holes Moine Thrust • The Moine Thrust occurred during the Caledonian Orogeny in the late Ordovician to early Silurian (455-430 Ma) after the continents collided and the Iapetus ocean closed at 420Ma • The Moine was lifted and thrust over the ancient Lewisian Gneiss on the Laurentian Plate as the European plate moved westwards • The thrust moved about 50 miles from east to west over a period of several million years and has a 10-20 degree dip to the east • The thrust zone is 120 miles long running from the north coast to the Island of Skye and has a maximum width of 6 miles Our Guides Donald and Wendy Fisher First Evening Social LOCATIONS IN NORTH SCOTLAND • Loch Glencoul • Smoo Cave • Traigh Allt Chailgeag Beach • Ben Arnabol Thrust Fault • Scourie Dikes • Cambrian Pipe Rock The Glencoul Thrust, Loch Glencoul Peach & Horne Memorial, Loch Assynt Dedicated to Ben Peach and John Horne who played the foremost part in unraveling the geological structure of the Northwest Highlands, 1883-1897, placed 1930 CLASSIC NORMAL FAULT SMOO CAVE Traigh Allt Chailgeag Beach Traigh Allt Chailgeag Beach Ben Arnabol Thrust Fault Dunrobin Castle Geology Museum Red Garnets in Green Norwegian Eclogite Scourie Dike Cambrian Pipe Rock LOCATIONS IN NORTH SCOTLAND • Loch Glencoul • Smoo Cave • Traigh Allt Chailgeag Beach • Ben Arnabol Thrust Fault • Scourie Dikes • Cambrian Pipe Rock • Knockan Crag • Strath Dionard • Stac Pollaidh • Clatchtoll – Split Rock • Achmelvich • Old Man of Steor • Bone Caves • Inverewe Gardens • Mt Slioch Knockan Crag, Northwest Highlands Geo Park The story of Peach and Horne’s 14 year study of the Highland’s geology is well told at Knockan Crag Statues of John Horne and Ben Peach at Knockan Crag Geological Park which John Horne and Ben Peach shows the various post Cambrian rock outside the Inchnadamph Hotel layers including the over riding Moine on Loch Assynt Schists Donald explaining the wonders of Knockan Crag Knockan Crag, Northwest Highlands Geo Park View of cliff side at Knockan Crag(~400’ high) showing 5 sedimentary layers covered with Moine Schist(quartzite, pipe rock, fuccoid beds, salterella grit, limestone) Knockan Crag, Northwest Highlands Geo Park Cambrian pipe rock, quartzite with Fucoid Beds, sandstone & siltstone & vertical worm holes vegetable matter cemented by carbonate Limestone over Fucoid Bed & Salterella Grit, Contact between the limestone and the schist sandstone, shell casts & carbonate Moine Schist over Durness Limestone, Knockan Crag Chirstine Marechal’s arms across 500 million years of time Mylonized Schist, Knockan Crag Top of Knockan Crag with Cul Beag(2538’) and Stac Pollaidh(2039) behind Imbricate Structure Strath Dionard • After a long 11 mile hike the peak Foinaven is seen which is formed by multiple thrusts of the same quartzite layer forming an imbrication • These thrusts are driven by the main Moine thrust to the west • The folding on the back side of the imbrication is very evident Assynt Geology • All of the rocks are old with most being precambrian and the youngest being the Silurean igneous intrusions (430 Ma) • The Assynt topography is intimately related to its geology • The Lewisean gneiss forms an undulating rocky plateau, varying from 500’ to 1000’ high and are dotted with lochs • The Torridonian sandstone forms detached mountains upon the plateau • The Cambrian quartzite are mountain builders since they are resistant to erosion and protect the rock underneath • The Moine schist to the east forms a large plateau but that is less rocky than the gneiss plateau to the west • Suilven (2400’) and Cul Mor(2800’) from Stac Pollaidh(2000’) showing 1400’ and 2000’ of Torridon sandstone over Lewisean Gneiss • At 800 million years ago the mountains were at their highest and it was estimated there was 17000’ of sandstone covering the gneiss • They have eroded back to their current heights leaving much of the area with only the original gneiss with isolated island mountains of sandstone View from Stac Pollaidh, 2013 Betty Elliot, Jennifer Truman, Brent Schaffer, Bud Elliott, Bonnie Voorhees, Virginia Adler Cross Bedding Torridon Sandstone, Stac Pollaidh Cross Bedding Torridon Sandstone Looking North, Stac Pollaidh Achnahaird,Hebridean Goat Split Rock, Clachtoll, 1Bya sandstone over 3Bya gneiss, strike N-S Clachtoll,unconformity between Lewisean Gneiss and Torridon Sandstone, Strike E-W indicating an ancient canyon Precambrian stromatolites on Clachtoll beach Clachtoll – Lazy Beds,2000Ya Clachtoll, 1 billion year fossilized waves in sandstone Clachtol, Steor Bay Clachtoll, Iron age Broch, 100yr BC to 100yr AD Stac Fada cross bedding in sandstone and deformation and darkening due to a meteorite impact. Thought until recently to be due to a lava intrusion Darker Material Ejecta From Meteorite Impact Gneiss Country, Achmelvich, Assynt Gneiss shore, Achmelvich Highly folded Lewisian Gneiss where the orange color is due to minerals containing iron and manganese More Gneiss Country, Achmelvich Achmelvich, contorted banding in gneiss Achmelvich, Scourie dyke, dolerite intrusion into the gneiss Canisp(2790’) and Suilven(2450’) looking east over the Lewisean gneiss from Achmelvich Steor Head Lighthouse Lighthouse at Point of Steor, Assynt Old Man of Steor,sixty meter(60m) column of Torridonian Sandstone Cutting Beds inCross Torridonian Sandstone (1 Billion Years Old) Trail to Bone Caves Bone Caves The Bone Caves were formed before the last ice age over 10,000 years ago by water eroding the limestone. Many bone remnants have been found including a walrus bone pin 2000 years old indicating people lived in the area at that time Bone Caves Pierre Marechal, Mary Springer, Bonnie Voorhees, Donald Fisher, Christine Marechal Mike Adler, Mike Scher, Nancy Ayers An Artesian spring Inverewe Gardens Jack Hales (the weatherman), Susan Hales Mt Slioch,